Much of the technical terminology to be used herein is to be understood to have the same meaning defined in Underdown et al. U.S. Pat. No. 4,443,347, namely:
(1) The terms "percent" and "parts" refer to percent and parts by weight, unless otherwise indicated.
(2) The term "resole" refers to a resin product of the partial condensation of a phenol with an aldehyde in such proportions that the partial condensate is capable of further condensation to an infusible or thermoset condition. Resoles can be either soluble or dispersible in water if condensation is stopped at an early stage or they can be essentially water-insoluble solids most often used in powdered condition. Inherently, resoles are fusible in the sense that, when heated to a sufficiently high temperature, they soften and will resolidify upon cooling; however, resoles are capable of undergoing further condensation when heated to a temperature sufficiently high that they fuse, particularly under either acid or alkaline conditions, so that they tend to undergo change when heated to fusion. Resoles are, most often, produced by the alkaline condensation of unsubstituted phenol (hydroxy benzene) with formaldehyde. The mole ratio of formaldehyde to phenol usually ranges from about 1.1:1 to about 1.5:1. The condensation is most often carried out in the presence of a fixed alkali, for example sodium hydroxide, calcium hydroxide or barium hydroxide and at a number of different temperatures, usually all in excess of about 100.degree. F. The chemical and physical properties of a resole can be varied within substantial limits, usually depending upon such factors as the mole ratio of aldehyde to phenol, the nature of the condensation reaction, what is done with the fixed alkali after completion of the condensation and the nature and proportion of any chemical additive that is present.
(3) The term "resite" refers to an infusible or thermoset resin material produced by the further condensation of a resole.
(4) The term "novolac" refers to the resin products of the substantially complete condensation of a phenol with an aldehyde in such proportions that condensation is not capable of proceeding to form an infusible product. Novolacs are usually produced by the condensation of unsubstituted phenol with formaldehyde in approximately equimolecular proportions, often with a slight excess of phenol. A novolac is fusible and, since it does not contain a sufficiently high proportion of formaldehyde to enable condensation to a thermoset condition, it can be heated to fusion and then resolidified repeatedly without undergoing chemical change. A novolac can be mixed with hexamethylenetetramine, or another formaldehydedonor, e.g., paraformaldehyde or a resole, and the mixture can then be condensed to a thermoset, or infusible, condition.
(5) "Conductivity" is the permeability of a proppant in Darcies or other suitable units multiplied by the fracture width measured in feet or other suitable units.
(6) The phrase "closure stress" refers to a pressure, read in pounds per square inch, applied to a given surface.
(7) "Sand" is a natural material, generally comprising quartz.
(8) The phrases "pre-cured" and "pre-cured proppant charge" refer to particles which have been substantially coated with a resin and the resin thereon cured to completion prior to introduction of the charge into a subterranean well.
It has been discovered that pre-cured resin-coated sand particles are extremely useful as proppants in oil wells, gas wells, water wells and other similar bore holes. These proppants provide particularly good performance at high closure stresses, e.g., from 6,000 to about 10,000 psi. Sand particles coated with cured phenolformaldehyde resin have proven particularly useful at these high closure stresses.
One method of preparing pre-cured proppant charges is disclosed in the commonly assigned Smillie U.S. Pat. No. 4,494,318. In the method described in that application, a single layer of uncured resin-coated particles is fed to a heated rotating surface. As the resin-coated particles are carried on the rotating surface, they are cured by the heat of the surface. The cured resin-coated particles are removed from the rotating surface by a scraper blade which contacts the rotating surface at a predetermined angle of surface rotation.
Other methods of preparing pre-cured proppant charges are described in the aforementioned Underdown U.S. Pat. No. 4,443,347 and in Johnson et al. U.S. Pat. No. 4,439,489.